1. Field of the Invention
The present invention relates to a semiconductor device formed from a semiconductor film that has a crystal structure as well as a method of manufacturing the same, and more specifically, to a semiconductor device including a field effect transistor whose channel formation region is formed from a crystalline semiconductor film on an insulating surface and a method of manufacturing the semiconductor device.
2. Description of the Related Art
A technique of forming an amorphous semiconductor film on a glass or other insulating substrate and crystallizing the film by laser light irradiation is known. A thin film transistor (hereinafter referred to as TFT) manufactured from a semiconductor film having a crystal structure (crystalline semiconductor film) is applied to planar display devices (flat panel displays), typically, liquid crystal display devices.
Laser light is applied to a semiconductor manufacturing process in the form of a technique of re-crystallizing a damaged layer or an amorphous layer in a semiconductor substrate or a semiconductor film and in the form of a technique of crystallizing an amorphous semiconductor film formed on an insulating surface. Laser oscillators usually used are gas lasers represented by excimer lasers and solid-state lasers represented by YAG lasers.
An example of crystallizing an amorphous semiconductor film by laser light irradiation is disclosed in JP 62-104117 A. In this example, the laser light scanning speed is set to the beam spot diameter multiplied by 5000 per second or faster to make an amorphous semiconductor film into a polycrystalline film through high-speed scanning without melting the film completely. Another known example is to use laser processing apparatus disclosed in JP 08-195357 A and process laser light into a linear beam by an optical system before irradiation.
JP 2001-144027 A discloses a technique of manufacturing a TFT in which a solid-state laser oscillator such as an Nd:YVO4 laser is employed to irradiate an amorphous semiconductor film with the second harmonic of its laser light and to form a crystalline semiconductor film of larger grain size than prior art.
However, the mainstream method for forming on an insulating surface a high quality crystalline semiconductor film which has less defects and grain boundaries, or sub-grain boundaries and which is less fluctuated in orientation has been re-crystallization of a semiconductor film on a single crystal substrate after the film is heated and melted at high temperature, which is known as a zone melting method.
It is considered that the problem is that the method utilizes level differences of a base as in the known graphoepitaxy technique and crystals grow along the level differences to leave level differences on the surface of the obtained single crystal semiconductor film. In addition, a single crystal semiconductor film cannot be formed by graphoepitaxy on a glass substrate which has a relatively low distortion point.
On the other hand, when an amorphous semiconductor film formed on a flat surface is crystallized by laser light irradiation, polycrystals are obtained and defects such as grain boundaries are formed at random. Therefore crystals having the same orientation cannot be obtained.
A grain boundary has a large number of crystal defects, which serve as carrier traps and are considered as the cause of lowering in mobility of electrons or holes. It has been impossible to form a semiconductor film that has no defects, grain boundaries, or sub-grain boundaries caused by volume shrinkage of a semiconductor, thermal stress with a base, and lattice mismatch which accompany crystallization. Accordingly, it has been impossible for a crystalline semiconductor film formed on an insulating surface by crystallization or re-crystallization to achieve the quality of a MOS transistor formed on a single crystal substrate without bonding SOI (Silicon on Insulator).
For instance, when a semiconductor film is formed on a glass substrate to build a TFT, the TFT is arranged giving no regard to randomly-formed grain boundaries and therefore the crystallinity of a channel formation region of the TFT cannot be controlled strictly. The randomly-formed grain boundaries and crystal defects lower the characteristics and cause fluctuation in characteristic between elements.